This invention relates to a method for contactless knurling of high modulus thermoplastic films, such as polyester and polyimide. In a preferred embodiment, fast drying ink is intermittently deposited onto the margin of the moving film surface prior to winding. In another preferred embodiment, a laser is employed to intermittently modify the margin of the moving film surface prior to winding.
The manufacture of high modulus thermoplastic film webs is well known. For example, polyethylene terephthalate film is made by first extruding molten polyethylene terephthalate resin through a slotted die to form a cast melt of polyethylene terephthlate. The melt is quenched on a casting drum to produce a cast sheet. The cast sheet is biaxially stretched in the longitudinal and transverse directions to form a film. The amount of stretching imparted to the film in these two perpendicular directions is a function of the desired tensile properties that the manufacturer wishes to impart to the film. The biaxially drawn film is thereafter heat set at elevated temperatures to relax and crystallize the film and thus impart to the film the desired thermal stability. The film is immediately cooled after heat setting and, after trimming the margins, is wound on to a mill or master roll. The master roll is usually slit into standard width films. These films are rolled to form "slit rolls." In most cases, the slit rolls are stored and/or shipped prior to use.
For many applications the surface characteristics of a thermoplastic web are critically important. Perhaps equally important is the uniformity of the film thickness and the uniformity of its surface characteristics. Unfortunately, the high speed winding operations performed at the end of high modulus film manufacture can degrade the quality of the film due, in part, to the sheet deformation and surface scratching associated with high modulus films such as polyesters and polyimides.
When a thermoplastic web such as polyethylene terephthalate is being wound, the goal is to produce an "ideal film roll", defined as a perfect right circular cylinder having a firm, uniform pressure distribution throughout the roll. The individual film layers of an ideal film roll are stacked so that their edges present flat and smooth surfaces on both ends of the roll. The face of an ideal film roll does not exhibit the commonly encountered "non-ideal" web characteristics such as "gauge bands", "cross buckles", "honeycomb", "pimples", "shifted layers", and "telescoping". These terms are defined below:
Gauge Bands - Variations in web thickness as measured transverse to the "machine" or length direction, of the film web which induce non-uniform pressures in a wound roll of film.
Cross-buckles - Patterns of flat or non-uniform curvature on the surface of a roll of film extending along axial direction and accompanied by corresponding pressure gradients in the circumfirential and radical directions.
Honeycomb - Patterns of non-uniform smoothness and pressure distribution resembling chicken wire. This pattern can exist on the surface or be hidden below the surface layers. It can occur at the time of winding or develop with time as entrained air escapes to reveal a non-uniform pressure distribution.
Pimples - Localized hard spots, generally round in shape, and varying in height and diameter according to severity. Sometimes originated by winding a particulate contaminate between layers of film. Subsequent layers often tend to distort due to the localized non-uniform pressure resulting in the "Pimple" defect.
Shifted layers - Term applied to individual or groups of film layers whose margins protrude from the edge surface of a roll of film. Shifted layers often occur due to a momentary pulse in the winding tension magnitude or direction.
Telescoped roll - A roll of film having a portion of its layers systemmatically shifted axially away from a perfectly stacked configuration. This defect can occur during winding or during handling subsequent to winding. It predominates in rolls of film wound with low radial pressures and having low surface friction. A smooth and uniformly tapered crossweb thickness profile increases the tendency for a roll to telescope during winding.
Painstaking process control, especially with respect to film width gauge uniformity, can eliminate or significantly reduce the occurrance and severity of the non-ideal film web characteristics listed above. Post-winding heat treatment, as described in U.S. Pat. No. 3,873,664, can also minimize sheet deformations due to winding of high modulus thermoplastic films.
Film manufacturers have also intentionally modified the film surface to improve the winding characteristics of the film. In particular, the outermost (1/8 to 1/2 inch wide) margins of the film web have been embossed ("knurled") to prevent the "shifted layers" phenomenon. Contact knurling is typically produced by means of a circular metal roller in rotating contact with the margin of the moving film web. The roller typically has small protrusions facing radially outward and regularly spaced about the circumerference of the roller. A second, rubber-faced roller is in rotating contact with the opposite film surface directly under the knurling roller. A line of spaced indentations is produced in the film margin as it passes the rotating knurling roller. These identations serve to minimize the shifted layers phenomenon during winding by lining up and engaging with the indentations of previously wound film. Perfect alignment of the indentations is impossible due to the knurling patterns selected and the increased radius of each succeeding layer. The knurled indentations which do not firmly engage themselves with an earlier wound film surface indentation create passageways by which entrained air may escape from the film roll. They also form a region of high pressure contact at the margins of a roll. This promotes conformity of web surfaces within the roll during winding.
The above-described contact knurling process is acceptable for relatively thick high modulus films (0.002 inch and above); it is unsuitable for thinner gauge films. The rotating knurling and rubber wheels typically introduce a stress gradient across the width of the film. While this stress gradient does not adversely affect a thick film, such stress can significantly deform a thin film, thereby creating additional winding problems. Other problems with knurling include debris formation and contamination of the film web surface, and the necessity for precise control and alignment of the knurling apparatus.